The present invention relates to a propylene-ethylene copolymer composition and a process for producing the same. More particularly, it is concerned with a propylene-ethylene copolymer composition suitable for use as film for insulating oil-impregnated capacitors and also to a process for producing said composition.
Recent years have been the wide spread use of biaxially-stretched laminated film composed mainly of polypropylene (PP for short) and having a roughened surface on at least one side thereof, into the fields of electric, packaging, and printing industries.
Among the applications of film with a roughened surface is film for insulating oil-impregnated capacitors. According to a recently proposed process, it is produced, for example, by the steps of forming, by melt processing, a PP sheet with the .beta.-type crystalline structure from a raw material incorporated with a specific nucleating agent, and stretching the PP sheet, thereby roughening the film surface.
In the fields of packaging and printing, there has been proposed a new process for producing film with a roughened surface by biaxially stretching a laminate film having, on at least one side thereof, a layer of propylene-ethylene copolymer composition. This process efficiently provides a film with a high haze value (ASTM D-1003), which is a measure to indicate the degree of surface roughness of film and with easiness of processing.
PP films with a roughened surface should meet the following requirements, if they are to be used as a film for insulating oil-impregnated capacitors.
(1) They should have a high breakdown voltage. PA0 (2) They should permit the insulating oil to infiltrate easily into the space between the dielectric material and the electrode surface. In other words, they should have a high haze value. PA0 (3) They should have high oil resistance for the insulating oil.
Unfortunately, films obtained from the sheet with the .beta.-type crystalline structure do not meet these requirements, because they lack fine and uniform surface roughness, so that they do not have the desired surface roughness and also do not permit the rapid infiltration of insulating oil.
By contrast, the biaxially stretched laminate film with a rough surface of propylene-ethylene copolymer layer, which is conventionally used in the fields of packaging and printing, has a high haze value and hence permits the rapid infiltration of insulating oil. However, such a laminate film is poor in oil resistance (or it contains a large amount of fraction which is extracted by insulating oil), because it has a low isotactic index (II for short), which is expressed in term of the residual amount remaining unextracted when extracted with boiling n-heptane for 10 hours. In addition, the tan .delta. of insulating oil (measured according to JIS C2320, 80.degree. C./p-xylene, 2 hr) and also the viscosity of insulating oil are increased, adversely affecting the performance of capacitors.